Techniques of bonding a hologram to a voucher or a credit card or integrally forming a voucher or a credit card with a hologram for forgery prevention are known.
With a known technique, a hologram is prepared by dividing a laser beam into two beams and irradiating one of the laser beams onto an object to be imaged that provides the basis of a three-dimensional image so as to make scattered and reflected light thereof get to a hologram recording material and not irradiating the other laser beam onto the object to be imaged but causing the other laser beam to directly get to the hologram recording material such that the light beams following the two different routes interfere with each other on the hologram recording material and finally recording the interference fringes produced as a result on the hologram recording material. With such a technique, however, a real object to be imaged needs to be provided and hence it is not possible to prepare a hologram of an object to be imaged that cannot be provided as real object (such as character strings floating in air).
On the other hand, computer generated holograms (CGHs) have recently been finding practical applications. A CGH is prepared by providing an object to be imaged with shape and material data for a three-dimensional CG (computer graphics), generating a pattern of interference fringes by means of a computer simulation as in the case of the interference fringes recorded on a hologram recording material by a known hologram preparing method, and micro-fabricating the generated pattern of interference fringes. Since a hologram of an object can be prepared by a technique of preparing a CGH even if the object cannot be provided as real object, CGHs are attracting attention as holograms having a high anti-forgery effect (see Patent Document 1).
While a three-dimensional image that is hardly blurred all the way from the front side to the rear side of a hologram can be observed when the hologram is irradiated with light from a point light source or with parallel rays of light, the image is blurred both at the front side and at the rear side of the hologram particularly at points distant from the hologram so that a clear three-dimensional image cannot be observed when the hologram is irradiated with light from a large light source (that produces a wide range of incident angle of light striking the hologram).
On the other hand, holograms are employed for anti-forgery applications and design applications because there is a demand for strange appearances of holograms that a deep three-dimensional image can be observed by means of a physically two-dimensional hologram medium.
The effect of a strange appearance of a hologram is emphasized when a deep three-dimensional image can be observed. However, as pointed out above, the image is blurred both at the front side and at the rear side of the hologram particularly at points distant from the hologram when the hologram is irradiated with light from a large light source. Therefore, a contrary result is produced when an infinitely deep three-dimensional image is achieved.
Techniques of preparing a computer generated hologram by making a three-dimensional image 10′ project frontward and rearward by a substantially same quantity relative to the hologram recording plane 20 as shown in FIG. 22 have been practiced in order to produce a deep three-dimensional image with a minimized level of blur (see Patent Document 2).